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Full text of "The manufacture and properties of iron and steel"

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THE BLAST FURNACE.                                       63
the pores of the oxide, and this carbon remains associated with the iron, finally furnishing the proportion needed for its conversion into pig-iron. This carbon deposition begins at a temperature of about 300 C. (570 F.), soon after.the first stages of reduction are under way, rapidly increases until all the Fe203 is reduced to Fe304 at a temperature of about 450 C. (840 F.), and then continues at a slower rate until the Fe304 is all reduced to FeO at a temperature of about 600 C. (1110 F.). The mixture of carbon and metallic iron descends until the zone of fusion is reached, when the mixture is converted into iron carbide.
As above stated, the gases reduce the Fe203 and at a temperature of 450 C. the iron is nearly all present as Fes04. This descends unchanged until at 13^ feet it meets a temperature of 500 C. (930 F.), when it is strongly acted upon and converted into FeO, the transformation being complete when a temperature of about 580Q C. (1080 F.) is reached at a depth of 19 feet. This FeO so formed, impregnated with deposited carbon, descends quite a distance unchanged until a temperature of 700 C. (1290 F.) is encountered at a depth of 26 feet, when the last atom of oxygen is taken by the carbonic oxide', and spongy iron begins to form. This reaction is completed when the temperature reaches 800 C. (1470 F.) at a depth of 32 feet.
The limestone comes down through the furnace until it encounters the temperature of 800 C. (1470 F.), at which the last of the FeO is reduced to spongy iron, when it is decomposed and the carbonic acid is driven off to rise through the stock, while caustic lime (CaO) descends to-the zone of fusion to flux the silicious ingredients of the charge. The carbonic acid (C02) from the limestone plays an important and objectionable part in its passage to the tunnel head. At all temperatures above 550 C. (1020 F.) the following reaction occurs:
C02+C=2 CO,
and as the limestone is not decomposed until a temperature of 800 C. is reached, it follows that during the passage of this carbonic acid from the point where it is made at a depth of 32 feet until it reaches a temperature of 550 C. (1020 F.) at a depth of about 17 feet, which is to say, during the travel of the gas through